1. Technical Field.
The present invention relates to self-propelled paving construction equipment and more particularly to slip form pavers in which flowable paving material is continually molded in a pre-determined cross-sectional shape along the ground and to a control system therefor.
2. Background Information.
Self-propelled slip form paving machines are generally well known and can be used to form curbs, gutters, spillways, sidewalks, troughs, barriers, and other continuous extrusions from concrete or other paving materials. These machines generally include a main frame supporting an operator station as well as the propulsion, hydraulic, and control systems. The main frame is often supported on tracked members by extendable/retractable posts. The main frame also supports a mold having a shape corresponding to the desired cross-sectional shape of the structure to be formed and a mold hopper for receiving paving material from a reservoir of paving material, which is often carried by a separate truck traveling adjacent to the paving apparatus. Paving material is often conveyed to the mold hopper by means of a rubber belt conveyor or spiral auger conveyor apparatus. Positioning of the mold during paving operations is usually accomplished by steering the tracked members and by extending or retracting the posts supporting the main frame, which changes the position of the main frame and therefore changes the position of the attached mold.
It is also known to automatically control movement of self-propelled slip form pavers using an external datum such as a string line and a plurality of sensors. A string line is carefully positioned using ground stakes, line rods, and line holders such that the string line is positioned at a known distance and elevation away from the desired location of the paved structure.
Once a string line has been prepared, the slip form paver can be positioned adjacent to the string line. A steer sensor, often consisting of a vertical wand attached to an electrical device that generates an electrical output signal proportional to the movement of the vertical wand away from a neutral or "null" position, is extended from the paver toward the string line such that the steer sensor wand is in contact with the string line and in the neutral position when the mold on the paver is in the desired location. A grade sensor, often consisting of a horizontal wand attached to an electrical device that generates an electrical output signal proportional to the movement of the horizontal wand away from a neutral or "null" position, is also extended from the paver to the string line such that the grade sensor wand is in contact with the string line and in the neutral position when the mold is in the desired position. Often, more than one grade or steer sensor is used on a given paver.
It should be noted that the term "grade" as used herein refers to change in level of the ground surface in the direction of paver travel. A paver traveling "uphill" therefore is traveling up a grade. On the other hand, the term "slope" as used herein refers to the change in ground level across the path of paver travel and is determined by the angle of the ground surface across the path of paver travel relative to an imaginary horizontal plane. A paver traveling over a slope, therefore, tilts in a direction transverse to the direction of paver travel. Both grade and slope are conventionally measured in terms of percentages. For example, a one foot vertical rise in ground level over a road 100 feet wide would result in a slope of one percent (1%).
Once the steer sensor and the grade sensor, or the multiple steer and grade sensors if more than one of each are used on a specific paver, are correctly positioned on the string line, then the slip form paver may be automatically made to travel along the string line using a control system in which signals from the steer sensor are used to adjust the steering of the paver and signals from the grade sensors are used to adjust the posts connecting the main frame to the tracked members on the side adjacent to the string line. Often, a front grade sensor attached to the forward part of the frame and a rear grade sensor attached to the rear portion of the frame relative to the direction of paver travel will be used. In this case, the front grade sensor signal is used to induce movement of the front grade post and the rear grade sensor signal is used to induce movement of the rear grade post.
While controlling a slip form paver using only steer and grade sensors may be adequate to automatically position a paved structure at a desired location on level ground, these sensors are generally inadequate to satisfactorily position the paved structure when the ground over which the paver travels is sloped. In recognition of this problem, it is known in the art to provide a slope sensor on slip form pavers. Typically, a slope sensor consists of a dampened pendulum that produces an electrical signal proportional to any deviation of the pendulum from a vertical orientation. The output signal from a slope sensor is often used to induce movement of the post or posts connecting the frame to the tracked members on the side of the frame opposite the string line, which are referred to as the "slope posts." When the paver travels over a path that slopes downward from left to right, when looking at the rear of the paving machine, then the slope sensor generates an output signal used to extend the slope post on the right side of the paver to return the paver frame, and thereby the mold, to a level position.
Automatic control of slip form pavers is therefore known in the art. Once the paver is correctly positioned relative to the string line, it can begin automatic paving operations using a combination of steer, grade and slope sensors. If the paver moves away from the string line in the horizontal direction, then this movement is detected by the steer sensor, which generates an output signal used to steer the paver back toward the string line. If the elevation of the forward or rear portions of the paver deviates relative to the elevation of the string line, then this deviation is detected by the forward or rear grade sensors, which transmit electrical signals used to extend or retract the forward or rear grade posts. If the paver travels over a sloped path, then the slope sensor generates an electrical signal used to extend or retract the slope post. Because the mold is attached to the paver frame, the position of the structure formed by the mold is determined by the position of the paver frame with respect to the string line.
It is also known in the art to form a paved structure having a cross slope relative to the slope of the ground surface on which the structure is formed. In this respect, the term "cross slope" refers to the transverse angle of the paving mold relative to the ground surface. For example, it is often desirable to form a curb and gutter structure in which the angle of the top surface of the gutter increases relative to the ground surface as the gutter extends away from the curb to form a so-called "catch angle." Conversely, it may be desirable for the angle of the top gutter surface to decrease as the gutter extends away from the curb to form a so-called "spill angle." If the mold is rigidly attached to the paver frame, then changing the transverse angle of the paver frame with respect to the ground changes the cross slope of the mold, and hence of the paved structure formed by the mold.
In conventional slip form pavers, it is known to use the slope post and a remote slope setpoint device to change the cross slope of a mold. A remote slope setpoint device, which is typically a handheld potentiometer, can be used to introduce an error signal into a conventional paver control system that corresponds to a desired mold cross slope. Upon receiving such an error signal, the paver control system extends or retracts the slope post until the signal received from the slope sensor matches the error signal generated by the remote slope setpoint device. After this point, automatic paver operations continue as described above and the slope sensor signal is used by the control system to maintain the desired mold cross slope as the ground slope changes.
But using a remote slope setpoint device and a conventional paver control system is problematic when changing the mold cross slope during paver operation to form a paving structure having a variable cross slope. This is because extending or retracting the slope post as the paver automatically guides on the string line to change the mold cross slope also changes of the position of the mold relative to the string line. Such a change in mold position when changing cross slopes in conventional control systems is often unacceptable because many paving projects have specifications requiring accuracy in mold placement plus or minus a fraction of an inch over ten linear feet, which is usually far less than the mold movement generated using a remote slope setpoint device and an existing paver control system as described above to form a paving structure having a variable cross slope. Accordingly, the mold position changes must be manually compensated for by either adjusting the grade and steer sensor mounting jacks or by calculating the amount of elevation and alignment error induced during mold cross slope transition and then incorporating corrections for the calculated error into the string line setup. These manual compensation methods are time consuming and often difficult to accurately perform.
As shown by the above discussion, what is needed in the art is an automatic control system for a paving apparatus that allows for the automatic forming of structures in which the cross slope can vary without changing the relative position of the slip formed structure to the string line. Moreover, the need is for such a control system to be effective on both level ground and on ground in which the slope changes as the paver travels along its intended path. Such a control device would ideally also accommodate the use of steer and grade sensors such that paving may be accomplished completely automatically using an external datum such as a string line.